The present invention relates to the use of lasers to produce acoustic signals in liquid media, and more specifically, it relates to systems for diagnosing the presence of a gas bubble in liquid media.
In U.S. Pat. No. 4,986,659, titled xe2x80x9cMethod For Measuring The Size And Velocity Of Spherical Particles Using The Phase And Intensity Of Scattered Light,xe2x80x9d an improved apparatus and method for determining the change in the effective cross-section of a sample volume defined by two crossed laser beams is disclosed. A laser generation means is provided for generating a pair of coherent laser beams and means are provided to change the separation, intersection angle, and focused diameter of the beams. These beams are directed along an axis, and are caused to cross the axis at a given angle to define an interference pattern constituting a sample volume. A collection apparatus for sensing the scattering of light caused by particles, droplets, bubbles, or the like within the sample volume is provided. In the presently preferred embodiment, the collection apparatus is disposed at preferred off-axis angles including off-axis backscatter with the angle predetermined, and the angle defined by the direction of beam propagation. The collected scattered light is directed onto photo-detectors which are coupled to a signal phase determining means, for measuring the relative phase between the signals produced by each photo-detector and a signal amplitude determining means to measure the relative amplitude of the signals produced as the particle, drop, bubble, or the like passes through the sample volume. Sizing means are coupled to the signal phase and amplitude determination means for determining the size of the particle, drop, bubble, or the like from phase and amplitude changes in the received signals. Methods and apparatus are disclosed for determining the change in the effective cross-section of the sample volume due to size variations of particles passing through the interference pattern. The velocity of the particle drop, bubble, or the like is determined using well known laser Doppler anemometry techniques.
U.S. Pat. No. 5,263,361, titled xe2x80x9cApparatus For Leak Testing A Fluid Containing Chamber Utilizing A Laser Beamxe2x80x9d is directed to a method and apparatus for leak testing a fluid containing chamber wherein the chamber is pressurized with a gas and is submerged in a liquid. The bubbles of gas rising from the submerged chamber are directed past a plurality of a predetermined locations that are each in optical communication with a photoelectric detector. The signals from the detectors are counted and when the number of bubbles exceeds a predetermined number, a signal is activated indicating a leaking container. By grouping a number of adjacent photoelectric detectors into a predetermined set, the apparatus can discriminate between random bubbles rising from the chamber surface as it is submerged and a number of bubbles all originating from a given location indicating a leak. The photoelectric detectors may be positioned in the liquid adjacent the predetermined locations or positioned out of the liquid and coupled to the predetermined locations by fiber optic cables. Alternatively, a laser beam can be directed across the predetermined location and received by a detector on the opposite side of the laser source. When a bubble interrupts the laser beam, a signal is generated.
U.S. Pat. No. 4,662,749, titled xe2x80x9cFiber Optic Probe And System For Particle Size And Velocity Measurementxe2x80x9d discloses a system for the simultaneous measurement of the size and velocities of bubbles or drops in a multiphase process environment wherein light passing through a Ronchi grating is projected onto a measurement volume within the multiphase process stream by a coherent fiber optic bundle and a gradient index imaging lens. Drops or bubbles passing through the measurement volume reflect or refract light which is sensed by velocity and size sensor fiber optic bundles disposed opposite the imaging lens and the sensed signal is coupled to signal processing means which convert the light signal to electrical signals. The appropriate size velocity measurements are made using one or more of the visibility techniques, phase lag techniques or transit time techniques.
U.S. Pat. No. 5,473,136, titled xe2x80x9cMethod And Apparatus For The Machining Of Material By Means Of A Laserxe2x80x9d discloses a method for the machining of material using a laser with detection of the material to be machined, where laser light is directed at the material via a laser optical system and the light re-emitted by the material is guided to a first detector arrangement which measures the intensity of the light and behind which there is connected an evaluation circuit for controlling the laser power or energy. The energy fed to the material via the laser optical system is measured, and the detector arrangement supplies to the evaluation circuit a display signal which indicates the beginning of the dielectric breakdown. The evaluation circuit reduces the power of the laser and/or interrupts the laser pulse if no display signal has as yet occurred at a predetermined time at which a predetermined energy was fed to the material.
It is an object of the present invention to provide an optical based method of detecting the presence of a vapor bubble.
It is another object of the invention to produce a signal which indicates the presence of a vapor bubble.
Still another object of the invention is to provide a feedback system for control of laser pulses used for bubble formation.
A light source such as a laser is coupled into an optical fiber and transmitted to the desired origin of bubble formation. The light reflected back into the distal fiber tip is monitored as it returns and is emitted out of the proximal end of the fiber. As a bubble forms at the distal end, the amount of reflected light increases as the index of refraction mismatch increases (nfiberxe2x88x92nair greater than nfiberxe2x88x92nliquid). This signal can yield information about the bubble and irradiated material such as time of bubble formation and collapse, size of bubble, absorption characteristics of the material, and mechanical characteristics of the material. This data can be used in a feedback control system for optimizing irradiation conditions. The invention may be used in a variety of applications including remote detection of cavitation or vaporization of target material as a result of laser irradiation. It may be used in hospitals in conjunction with laser based methods of stroke treatment and can be used for remote bubble detection in a variety of experiments where bubbles are formed, particularly at the end of an optical fiber.